<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }

        /*#test{background: red}
        .app{background: black}
        div{background: gray}


        #test{background: black !important;}
        .app{background: gray !important;}
        div{background: red !important;}*/

        [id="test"]{background: pink}
        .app{background: red}
    </style>
</head>
<body>
    <div id="test" class="app">app</div>
</body>
<script>
    // 样式表 --> 规则 --> 选择器+声明块

</script>
</html>